The purpose of this contract is to establish rodent production colonies and produce disease-free, genetically homogeneous and microbiologically defined F-344 rats and B6C3F1 hybrid mice for the NTP and NIEHS toxicology research and testing programs. Genetically defined inbred pedigreed animals associated with defined microflora will be used to establish pedigreed expansion colonies. Offspring from these pedigreed expansion colonies will be used to establish production colonies and to produce contractually specified number of each species at weekly intervals. Rodent production will be modulated to supply adequate number of animals for the NTP studies without excess production. The Environmental Toxicology Program of NIEHS under the National Toxicology Program (NTP) organizes and conducts a comprehensive interagency testing and research program focused on determining potential human health hazards due to exposure to chemicals. The toxicologic evaluation of chemical sis generally conducted through a sequence of experiments that involve acute, subacute, subchronic and chronic exposure of laboratory animals to chemical substances. The chronic rodent bioassay is the current preferred procedure for determining the carcinogenic potential of chemicals. Rats, mice and other small laboratory animals are appropriate species for evaluating toxicologic and carcinogenic properties of chemicals. Fischer 344 rats and B6C3F1 hybrid mice are the selected experimental animal species for evaluation of chemicals for toxicologic and carcinogenic properties under NTP. The centralized production facilities for F344 rats and B6C3F1 hybrid mice with homogeneous genetic properties, stable growth patterns and define microbial status will insure an adequate and continues supply of defined quality animals for the NTP and NIEHS toxicology research and testing program.